Terrible advice

I installed after market wheels onto my Nissan Navara which have the exact same spec (9Jx18 ET18) as the after market wheels supp**** by Nissan for the Navara. The tyre size used by the Navara for the OEM (7Jx18 ET45) wheel is 255/60R18 which has a diameter of 762mm and a circumference of 2395mm. I asked Nissan SA for some advice on the tyre size they would use on their after market wheel, hoping to come as close to the OEM tyres as possible and they advised me to use the 255/70R18. This did not look right to me so after a few hours of my own research I found out that the tyre size they advised has a diameter of 815mm and a circumference of 2558mm resulting in a difference of 7% and 6.8% over the OEM tyre and a speedo error of 6.3%. In actual fact the closest tyre one can get to the OEM tyre is the 265/60R18 tyre which has a diameter of 775mm and a circumference of 2433mm resulting in a difference of 1.7% and 1.6%. with a speedo error of 1.6%. Even a 285/60R18 tyre is closer to the OEM tyre than what Nissan advised me. When I pointed out their bad advice they threw back in my face that should I decide to change the tyre size it could afffect the vehicle's warranty. Only reason I am giving Nissan SA 2 stars is because they bothered to respond to my website query.